To solve the problems occurring at the high overall pressure ratios concerning the high pinion shaft speeds, a plurality of impellers are arranged in series in the stages following the low-pressure stages (first or second pinion shaft) beginning from the second or third pinion shaft, via the intermediary of a disk diffuser and of a return ring to at least one pinion shaft according to DE 42 34 739.
The drawback of this design is that no intercooling takes place after each impeller at higher pressure ratios of the stage groups.
The solution described in DE-OS 25 18 628, in which one pair of impellers each is arranged back to back on the pinion shaft, makes such intercooling possible, but it even leads to impairments in terms of rotor dynamics, because a great distance between the center of gravity of the overhanging rotor part and the pinion shaft bearing develops here due to the radial inlet nozzle arranged between the gearbox case and the impeller.